Swine influenza, seasonality, and the northern hemisphere

28 April 2009

I have received several questions about my assertion that the northern hemisphere is safe, at least until the fall, from an epidemic of A/California/09/2009 influenza virus. Here are two representative questions:

If I recall correctly, Spanish flu hit hardest during the summer and fall.

As for the swine flu, is it impossible (and why?) to see the current contagion spreading in the northern hemisphere over the following months?

Influenza is a seasonal infection in temperate climates. The flu season in the northern hemisphere is between January and April, although infections have been observed in December and May. In the southern hemisphere the flu season extends from May to September. May is almost upon us, and based on past observations this signals the end of influenza in the northern hemisphere. As flu season begins in the southern hemisphere, it is quite possible that A/California/09/2009 virus will circulate extensively.

Let’s see if we learn anything from the timing of two past pandemics. The 1918-19 pandemic began in the spring of 1918 with an outbreak of mild respiratory disease in soldiers at Fort Funston, Kansas. The infection spread to other military bases in the US and eventually to Europe. These infections were mild and did not receive much attention from the press. At the end of August 1918 a more virulent H1N1 virus emerged which caused high mortality (2.5% in the US, compared to 0.1% for typical influenza). The disease peaked in September and November. In the spring of 1919 a second disastrous wave of infections took place.

The H3N2 strain of influenza virus was responsible for a pandemic in 1968-1970. This virus originated in Southern Asia in the summer of 1968 and was responsible for sporadic cases of disease from September through December in the US, England, and Japan. Epidemic spread in these and other northern hemisphere countries began in December and ceased at the end of April 1969. In Australia epidemic spread of the H3N2 virus began in January 1969, ended in October, and recommenced in June 1970.

This history demonstrates the seasonality of pandemic influenza, and suggesta that spread of A/California/09/2009 in the northern hemisphere is not imminent. Based on this regularity, the epidemic in Mexico should be over no later than the end of May. While it is not ‘impossible to see the current contagion spreading in the northern hemisphere over the following months’, it would be unprecedented.

– “The flu season in the northern hemisphere is between January and April, although infections have been observed in December and May.”

– “At the end of August 1918 a more virulent H1N1 virus emerged which caused high mortality (2.5% in the US, compared to 0.1% for typical influenza). The disease peaked in September and November.”

– “This history demonstrates the seasonality of pandemic influenza, and suggests that spread of A/California/09/2009 in the northern hemisphere is not imminent. Based on this regularity, the epidemic in Mexico should be over no later than the end of May. While it is not ‘impossible to see the current contagion spreading in the northern hemisphere over the following months’, it would be unprecedented.”

Am I missing something? How can a summer pandemic be unprecedented? You cited a pretty famous example of one. In fact nearly all of your examples seem to have occurred partly or mostly “out of season”.

While we have data on influenza epidemics and pandemics in the Northern hemisphere, and some data on the early pattern of pandemics before they reached the Northern hemisphere, what has been the quality of the total surveillance data in the Southern hemisphere in the last century? Are we not drawing conclusions based on grossly incomplete data?

There are plenty of good data from the southern hemisphere, fromplaces such as Australia, which has very good reporting, and someother countries. So no, we are not drawing conclusions on grosslyincomplete data.

I answered Dave's question directly on the blog the other day – I takemany of the questions posted in comments and answer them in blogposts. In this way they are seen by more people.

Skeptigal

While we have data on influenza epidemics and pandemics in the Northern hemisphere, and some data on the early pattern of pandemics before they reached the Northern hemisphere, what has been the quality of the total surveillance data in the Southern hemisphere in the last century? Are we not drawing conclusions based on grossly incomplete data?

There are plenty of good data from the southern hemisphere, fromplaces such as Australia, which has very good reporting, and someother countries. So no, we are not drawing conclusions on grosslyincomplete data.

I answered Dave's question directly on the blog the other day – I takemany of the questions posted in comments and answer them in blogposts. In this way they are seen by more people.